Polypropylene carbonate-based solid polymer electrolyte

Polypropylene carbonate-based solid polymer electrolyte

Polypropylene carbonate (PPC) is a new type of degradable aliphatic polycarbonate obtained by the copolymerization of carbon dioxide and propylene oxide. It has many advantages such as biodegradability, low glass transition temperature, high thermal dimensional stability, and its structure is amorphous with high dielectric constant, and each repeating unit also has a very polar carbonate group. Therefore, structurally speaking, PPC is very suitable as a matrix for solid polymer electrolytes.

Inspired by the traditional Chinese culture of Tai Chi, Qingdao Energy Storage Industry Technology Research Institute proposed the design concept of “combining rigidity and flexibility” to design and prepare solid polymer electrolytes. That is, a rigid non-woven porous membrane is used to load a flexible polymer ion transport material polypropylene carbonate (PPC) to obtain a “rigid and flexible” composite solid polymer electrolyte (CPPC-SPE). Compared with the conventional PEO solid-state polymer electrolyte (PEO-SPE), the electrochemical stability window of this solid-state electrolyte can reach 4.6V at room temperature. The room temperature ionic conductivity of CPPC-SPE can reach 4.2×10-5S/cm, which is the highest reported solid polymer electrolyte with room temperature ionic conductivity. The reason is that: PPC has an amorphous structure and the molecular chain is prone to internal rotation. After the lithium ion is combined with the carbonyl oxygen on the main chain, the chain segment moves faster and the lithium ion migrates faster, so the ionic conductivity is higher. The room temperature rate charge-discharge and long-cycle performance of the solid polymer electrolyte were studied, and the results showed that: at room temperature, PEO cannot perform normal charge and discharge at 0.2C, while CPPC can complete fast charge and discharge at 0.1C, 0.2C and even 2C (Figure 1); in terms of cycle performance, the capacity retention rate was 95% after 1000 cycles at room temperature.

Polypropylene carbonate-based solid polymer electrolyte
Figure 1 – Rate cycling diagram of LiFePO4-Li half-cell assembled by CPPC-SPE at room temperature

Aiming to further optimize the electrochemical and other properties of PPC solid polymer electrolytes, the researchers developed an organic/inorganic composite solid electrolyte, and designed and constructed a PPC/LLZTO composite solid electrolyte by combining lithium lanthanum zirconium tantalum oxide (LLZTO) inorganic fast ion conductors. Through characterization, it was found that the ion migration number of the composite solid electrolyte was 0.75, and the electrochemical stability window reached 4.6 V. Combined with molecular dynamics simulation, the advantages of the organic-inorganic composite solid electrolyte were further proved.

In recent years, composite solid-state polymer electrolytes have shown good comprehensive properties in terms of ionic conductivity, mechanical properties, and electrochemical stability. It is common to add plasticizers in the method of preparing composite polymer electrolytes, such as organic solvents such as ethylene carbonate, propylene carbonate and dimethyl carbonate, but these organic solvents are easy to volatilize and burn, and there are potential safety hazards. Ionic liquids are organic liquids composed of only two ions, anion and anion, also known as molten salts at low temperatures. Ionic liquids have the advantages of low vapor pressure, electrical and thermal conductivity, wide temperature range in liquid state, and designability. It is precisely because of the incomparable properties of these other liquids that ionic liquids possess, Zhou et al. added ionic liquids (ILs) BMIM+BM4 to the PPC-based polymer electrolyte (Figure 2). The experimental data show that with the increase of BMIM+BM4 content, the glass transition temperature of the PPC-based composite polymer electrolyte gradually decreases, the thermal stability increases gradually, and the ionic conductivity also increases gradually. When the ratio of PPC/LiCIO4/BMIM+BM4 is 1:0.2:3, the room temperature ionic conductivity can reach up to 1.5mS/cm.

Polypropylene carbonate-based solid polymer electrolyte
Figure 2 – Digital photos (a) and visible absorption spectra (b) of BMIM+BM4-PPC-LiCIO4-BMIM+BM4- electrolytes with different mass fractions added

As a new type of solid polymer electrolyte matrix, PPC has high ionic conductivity and wide electrochemical stability window, which can meet its application in higher energy density lithium batteries and has great application prospects. However, as an electrolyte for energy storage devices, it also needs to be able to achieve fast lithium ion transport in different environments. Although PPC-based lithium batteries can be charged and discharged at room temperature, there is still a certain gap between their specific discharge capacity and liquid lithium batteries. Therefore, the room temperature ionic conductivity of the PPC solid polymer electrolyte can be further improved by adding inorganic particles or copolymerizing and blending. In addition, as an ester compound, it has poor chemical stability on the surface of an alkaline electrode material. When using this compound, care must be taken to reduce the alkalinity of the surface of the electrode material.